Systems and methods for using Stoneley waves for bottom-hole proximity detection

ABSTRACT

A downhole tool system may include a Stoneley wave emitter, located in a downhole tool, designed to emit Stoneley waves into a borehole. The downhole tool system may include one or more Stoneley wave sensors, located in the downhole tool, and a processor. The processor may be designed to receive signals from the one or more Stoneley wave sensors based on the detection of the Stoneley waves. The processor may use the signals to obtain a temporal measurement of the Stoneley waves. Based at least in part on the temporal measurement, the processor may calculate a distance from the downhole tool or a bottom-hole assembly to the bottom of the borehole.

BACKGROUND

This disclosure relates to using Stoneley waves in an acoustic loggingtool to monitor the borehole ahead of the bottom-hole assembly to get anaccurate measure of the distance to the bottom of the borehole.

This section is intended to introduce the reader to various aspects ofart that may be related to various aspects of the present techniques,which are described and/or claimed below. This discussion is believed tobe helpful in providing the reader with background information tofacilitate a better understanding of the various aspects of the presentdisclosure. Accordingly, it should be understood that these statementsare to be read in this light, and not as an admission of any kind.

Producing hydrocarbons from a borehole drilled into a geologicalformation is a remarkably complex endeavor. In many cases, hydrocarbonexploration and production may be informed by measurements from downholewell-logging tools and sensors that are conveyed deep into the borehole.The measurements may be used as part of the drilling process itself orto infer properties and characteristics of the geological formationsurrounding the borehole. During drilling, however, some parts of theprocess may not have applicable sensors suitable for downholeconditions. For example, the depth of a drill string may be determinedby a surface block position and a count of how many pipes have beenused. However, this method may lead to uncertainty in determining theposition of the drill string in relation to the bottom of the borehole.

As will be appreciated, once the drill string is in the borehole, it isnot continuously boring. On the contrary, the drill string is oftenpulled off the bottom of the borehole during other tasks, such as addinga new pipe or when conditions command some other downhole operation.However, relying simply on a pipe count and a surface block position maylead to considerable uncertainty and is susceptible to human error. Infact, when the drill string is lowered to the bottom of the borehole—anevent referred to as tagging bottom—great care is taken to minimize theimpact of landing and optimize the mechanics of resuming drilling. Thus,lowering the drilling string may be a very slow process to avoid damagedue to this uncertainty.

SUMMARY

A summary of certain embodiments disclosed herein is set forth below. Itshould be understood that these aspects are presented merely to providethe reader with a brief summary of these certain embodiments and thatthese aspects are not intended to limit the scope of this disclosure.Indeed, this disclosure may encompass a variety of aspects that may notbe set forth below.

To better enhance the measurement of the distance to the bottom of aborehole, Stoneley waves may be emitted and detected by a downhole tool.

Indeed, in one example, a downhole tool system may include a Stoneleywave emitter, located in a downhole tool, designed to emit Stoneleywaves into a borehole. The downhole tool system may also include one ormore Stoneley wave sensors, located in the downhole tool, and aprocessor. The processor may be designed to receive signals from the oneor more Stoneley wave sensors based on the detection of the Stoneleywaves. The processor may use the signals to obtain a temporalmeasurement of the Stoneley waves. Based at least in part on thetemporal measurement, the processor may calculate a distance from thedownhole tool or a bottom-hole assembly to the bottom of the borehole.

In another embodiment, a method may include emitting, from a downholetool, multiple Stoneley waves into a borehole in a pulse sequence. Themethod may also include detecting, via multiple sensors, the multipleStoneley waves traveling in the borehole. The method may also includecalculating, via a processor, a speed of the Stoneley waves in theborehole. The processor may also calculate a distance from the downholetool to the bottom of the borehole based at least in part on the speedof the Stoneley waves in the borehole and a time between detections ofthe plurality of Stoneley waves. Based at least in part on thecalculated distance from the downhole tool to the bottom of theborehole, the impact of the downhole tool with the bottom of theborehole may then be reduced or minimized.

In another embodiment, machine-executable instructions, stored on atangible, non-transitory machine readable storage medium, may beimplemented by a machine. When executed by the machine, the instructionsmay cause the machine to perform a method. The method may includecontrolling an emission of one or more Stoneley waves at multiplelocations in a borehole. The method may also include processing signalsreceived from one or more Stoneley wave sensors, wherein the signals arerepresentative of the one or more Stoneley waves sensed within theborehole. The method may also include calculating a direction of travelof the one or more Stoneley waves and a roundtrip time of the one ormore Stoneley waves propagating down the borehole and reflecting back tothe one or more Stoneley wave sensors based at least in part on thedirection of travel of the one or more Stoneley waves. The method mayalso include calculating a distance from a bottom-hole assembly to thebottom of the borehole based at least in part on the roundtrip time ofthe one or more Stoneley waves.

Various refinements of the features noted above may be undertaken inrelation to various aspects of the present disclosure. Further featuresmay also be incorporated in these various aspects as well. Theserefinements and additional features may exist individually or in anycombination. For instance, various features discussed below in relationto one or more of the illustrated embodiments may be incorporated intoany of the above-described aspects of the present disclosure alone or inany combination. The brief summary presented above is intended tofamiliarize the reader with certain aspects and contexts of embodimentsof the present disclosure without limitation to the claimed subjectmatter.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects of this disclosure may be better understood upon readingthe following detailed description and upon reference to the drawings inwhich:

FIG. 1 is a schematic diagram of a well-logging system including anacoustic tool, in accordance with an embodiment;

FIG. 2 is a schematic diagram of a drilling system including an acoustictool, in accordance with an embodiment;

FIG. 3 is an illustration of a bottom-hole assembly including anacoustic tool, in accordance with an embodiment;

FIG. 4 is a timing diagram of a wave propagating in a borehole, inaccordance with an embodiment;

FIG. 5 is a flowchart of a process for locating a bottom of a boreholeusing an acoustic tool, in accordance with an embodiment;

FIG. 6 is an illustration of a bottom-hole assembly including anacoustic tool with multiple detectors, in accordance with an embodiment;

FIG. 7 is a timing diagram of a wave propagating in a borehole, inaccordance with an embodiment;

FIG. 8 is a flowchart of a process for locating a bottom of a boreholeusing an acoustic tool, in accordance with an embodiment;

FIG. 9 is an illustration of a bottom-hole assembly including anacoustic tool at multiple depths within a borehole, in accordance withan embodiment; and

FIG. 10 is a flowchart of a process for locating a bottom of a boreholeusing an acoustic tool, in accordance with an embodiment.

DETAILED DESCRIPTION

One or more specific embodiments of the present disclosure will bedescribed below. These described embodiments are examples of thepresently disclosed techniques. Additionally, in an effort to provide aconcise description of these embodiments, the features of an actualimplementation may not be described in the specification. It should beappreciated that in the development of any such actual implementation,as in any engineering or design project, numerousimplementation-specific decisions may be made to achieve the developers'specific goals, such as compliance with system-related andbusiness-related constraints, which may vary from one implementation toanother. Moreover, it should be appreciated that such a developmenteffort might be complex and time consuming, but would be a routineundertaking of design, fabrication, and manufacture for those ofordinary skill having the benefit of this disclosure.

When introducing elements of various embodiments of the presentdisclosure, the articles “a,” “an,” and “the” are intended to mean thatthere are one or more of the elements. The terms “comprising,”“including,” and “having” are intended to be inclusive and mean thatthere may be additional elements other than the listed elements.Additionally, it should be understood that references to “oneembodiment” or “an embodiment” of the present disclosure are notintended to be interpreted as excluding the existence of additionalembodiments that also incorporate the recited features.

The oil and gas industry includes a number of sub-industries, such asexploration, drilling, logging, extraction, transportation, refinement,retail, and so forth. During exploration and drilling, boreholes may bedrilled into the ground for reasons that may include discovery,observation, or extraction of resources. These resources may includeoil, gas, water, or any combination of elements within the ground.

Boreholes, sometimes called wellbores, may be straight or curved holesdrilled into the ground from which resources may be discovered,observed, and/or extracted. The creation of a borehole may consist ofboring through a geological formation using a drill and a multitude ofsensors that measure and/or monitor the drilling process and logginginformation about the borehole. However, drilling may not be acontinuous process. On the contrary, the drill bit is often pulled offthe bottom of the borehole during tasks such as adding a new pipe orwhen conditions command some other downhole operation. When the drillstring is lowered to the bottom of the borehole (i.e., tags bottom)great care must be taken to minimize the impact of landing and optimizethe mechanics of resuming drilling. As such, an accurate measure of thedistance to the bottom of the borehole may improve safety and speed andreduce costs and inadvertent damages. In one example, the depth of thedrill bit on the end of a drill string may be determined by a surfaceblock position and a count of how many pipes have been used. However,the uncertainty with this type of measurement may be greater thandesired.

Another measurement uses an acoustic tool located within the borehole toestimate the distance from the drill bit to the bottom of the borehole.Additionally, the acoustic tool may be used in well-logging. Performedduring and/or after the formation of a borehole, well logging mayinclude making a detailed record of the geological formations penetratedby a borehole using one or more sensors. In well-logging, an acoustictool may generate and/or detect Stoneley waves, also known as tubewaves, to estimate properties of the borehole wall, such as fracturesand permeability.

Stoneley waves are boundary waves, also known as interface waves, whichpropagate along the wall of the borehole. While Stoneley waves generallyrefer to solid-solid interfaces, sonic measurements taken using Scholtewaves, which occur along a liquid-solid interface, or other sonic orultrasonic waves may also be used in certain embodiments. The physicalwaves, which may include vibrations, may propagate, reflect, andtransmit differently within the borehole depending on the physicalattributes of the borehole that the waves encounter. As such, Stoneleywaves may be used to estimate the location of the bottom of the boreholein relation to a tool near the drill bit, and, by extension, mayactively measure the distance between the drill bit and the bottom ofthe borehole.

With the foregoing in mind, FIG. 1 illustrates a well-logging system 10that may employ the systems and methods of this disclosure. Thewell-logging system 10 may be used to convey an acoustic tool 12 througha geological formation 14 via a borehole 16. In the example of FIG. 1,the acoustic tool 12 is conveyed on a cable 18 via a logging winchsystem 20 (e.g., vehicle). Although the logging winch system 20 isschematically shown in FIG. 1 as a mobile logging winch system carriedby a truck, the logging winch system 20 may be substantially fixed(e.g., a long-term installation that is substantially permanent ormodular). Any suitable cable 18 for well-logging may be used. The cable18 may be spooled and unspooled on a drum 22 and an auxiliary powersource 24 may provide energy to the logging winch system 20 and/or theacoustic tool 12.

Moreover, while the acoustic tool 12 is described as a wireline downholetool, it should be appreciated that any suitable conveyance may be used.For example, the acoustic tool 12 may instead be conveyed as alogging-while-drilling (LWD) tool as part of a bottom-hole assembly(BHA) of a drilling system (described below), conveyed on a slickline orvia coiled tubing, and so forth. For the purposes of this disclosure,the acoustic tool 12 may be any suitable downhole tool that usesStoneley waves within the borehole 16 (e.g., downhole environment).

As discussed further below, the acoustic tool 12 may receive energy froman electrical energy device or an electrical energy storage device, suchas the auxiliary power source 24 or another electrical energy source topower the tool. Additionally, in some embodiments the acoustic tool 12may include a power source within the acoustic tool 12, such as abattery system or a capacitor to store sufficient electrical energy toemit and/or receive the Stoneley waves.

Communications 26, such as control signals, may be transmitted from adata processing system 28 to the acoustic tool 12, and communications26, such as data signals, related to the results/measurements of theacoustic tool 12 may be returned to the data processing system 28 fromthe acoustic tool 12. The data processing system 28 may be anyelectronic data processing system that can be used to carry out thesystems and methods of this disclosure. For example, the data processingsystem 28 may include one or more processors 29, which may executeinstructions stored in memory 30 and/or storage 31. As such, the memory30 and/or the storage 31 of the data processing system 28 may be anysuitable article of manufacture that can store the instructions. Thememory 30 and/or the storage 31 may be read-only memory (ROM),random-access memory (RAM), flash memory, an optical storage medium, ora hard disk drive, to name a few examples. A display 32, which may beany suitable electronic display, may display images generated by theprocessor 29. The data processing system 28 may be a local component ofthe logging winch system 20 (i.e., on the surface), within the acoustictool 12 (i.e., downhole), a remote device that analyzes data frommultiple logging winch systems 20, a device located proximate to thedrilling operation, or any combination thereof. In some embodiments, thedata processing system 28 may be a mobile computing device (e.g.,tablet, smart phone, or laptop) or a server remote from the loggingwinch system 20.

As stated above, the acoustic tool 12 may also be implemented as part ofa drilling system 34, as illustrated in FIG. 2. The drilling system 34may be used to drill a borehole 16 into a geological formation 14. Inthe drilling system 34, a drilling rig 35 may rotate a drill string 36within the borehole 16. As the drill string 36 is rotated, a drillingfluid pump 37 may be used to pump drilling fluid, which may be referredto as “mud” or “drilling mud,” downward through the center of the drillstring 36, and back up around the drill string 36, as shown by referencearrows 38. At the surface, return drilling fluid may be filtered andconveyed back to a mud pit 39 for reuse. The drilling fluid may traveldown to the bottom of the drill string 36 known as the bottom-holeassembly (BHA) 40. The drilling fluid may be used to rotate, cool,and/or lubricate a drill bit 42 which may be a part of the BHA 40. Thefluid may exit the drill string 18 through the drill bit 42 and carrydrill cuttings away from the bottom of a borehole 16 back to thesurface.

The BHA 40 may include the drill bit 42 along with various downholetools (e.g., acoustic tool 12). The downhole tools, such as the acoustictool 12, may collect a variety of information relating to the geologicalformation 14 and the state of drilling in the borehole. For instance,the downhole tools may be LWD tools that measure physical properties ofthe geological formation 14, such as density, porosity, resistivity,lithology, and so forth. Likewise, the downhole tools may bemeasurement-while-drilling (MWD) tools that measures certain drillingparameters, such as the temperature, pressure, orientation of the drillbit 42, and so forth.

The downhole tools, such as the acoustic tool 12, may collect a varietyof data that may be stored and processed in the BHA 40, or be sent viacommunications 26 to the surface for processing via any suitabletelemetry (e.g., electrical signals pulsed through the geologicalformation 14 or mud pulse telemetry using the drilling fluid). As withthe well-logging system 10, a data processing system 28 may include aprocessor 29, memory 30, storage 31, and/or a display 32. As such, thememory 30 and/or the storage 31 of the data processing system 28 may beany suitable article of manufacture that can store the instructions. Thememory 30 and/or the storage 31 may be ROM memory, random-access memory(RAM), flash memory, an optical storage medium, or a hard disk drive, toname a few examples. The display 32 may be any suitable electronicdisplay that can display the well-logs and/or other information relatingto properties of the borehole 16 as measured by the downhole tools(e.g., acoustic tool 12). It should be appreciated that, although thedata processing system 28 is shown by way of example as being located atthe surface, the data processing system 28 may be located in the BHA 40,remotely off-site, or at any location suitable for the application.

As stated above, the acoustic tool 12 may be implemented as part of abottom-hole assembly (BHA) 40, as depicted in FIG. 3. The BHA 40 mayinclude a drill bit 42 and the acoustic tool 12. Additionally, theacoustic tool 12 may include a Stoneley wave sensor 44 (e.g., anysuitable acoustic wave detector, such as an acoustic transducer) and aStoneley wave source 46 (e.g., any suitable acoustic wave generator,such as an acoustic transducer). Stoneley waves are emitted by the wavesource 46 in a continuous or pulsed fashion, and travel along theborehole wall and are sensed by the wave sensor 44. In some embodiments,the continuous emission includes a train or sequence of multiple pulses.Based on the wave sensor measurement, the distance between the acoustictool 12 and the bottom of the borehole 48 may then be calculated.

FIG. 4 depicts a time diagram 50 showing the progression of the Stoneleywaves in time. At time 52 a Stoneley wave pulse is produced at theStoneley wave source 46. Due to the physical nature of the borehole 16and Stoneley waves, the Stoneley wave emission may include two pulses,one traveling up the borehole 16 towards the surface and one travelingdown the borehole 16 towards the bottom of the borehole 48. As depictedat time 54, the downward traveling Stoneley wave pulse may be detectedby the wave sensor 44. After passing the wave sensor 44 the Stoneleywave pulse travels along the wall of the borehole 16 until it reachesthe bottom of the borehole 48. Upon reaching the bottom of the borehole48, the Stoneley wave pulse may be at least partially reflected due tothe change in interface as shown at time 56. This partial reflection mayreduce the amplitude of the Stoneley wave by creating other waves. Forexample, the interface may cause parts of the Stoneley wave to beconverted to compressional and/or shear waves that may then be lost inthe volume of the borehole 16. Afterwards, at time 58, the reflectedStoneley wave pulse travels back up the borehole 16 to be detected asecond time by the wave sensor 44. Although the time diagram 50 depictsone Stoneley wave traveling through the borehole 16, it will beappreciated that multiple wave pulses, such as a wave train, may beused. As such, each subsequent pulse may be emitted before or after theprevious pulse has made its roundtrip to the bottom of the borehole 48.

In order to determine the distance from the drill bit 42 to the bottomof the borehole 48, the data processing system 28 may compute a temporalmeasurement of the time difference between the first detection of theStoneley wave pulse at time 54 and the second detection of the Stoneleywave pulse at time 58, thus defining a roundtrip travel time for theStoneley wave pulse. Multiplying the roundtrip time by the speed of theStoneley wave pulse may then yield an estimated distance between thewave sensor 44 and the bottom of the borehole 48. Stoneley waves withina borehole 16 generally propagate at approximately 1350 m/s (meters persecond), and this estimation may be used in the calculation. However,the speed of Stoneley waves can vary based on the conditions in theborehole 16 (e.g., formation porosity, tool placement, etc.). As such, aspeed approximation in a particular borehole 16 and position therein maybe used by dividing the distance between the wave source 46 and the wavesensor 44 by the time between emission, time 52, and the first detectionof the Stoneley wave pulse, time 54. Thus, the distance between the wavesensor 44 and the bottom of the borehole 48 may be calculated.Furthermore, the distance between the drill bit 42 and the wave sensor44 may be fixed and known prior to placing the BHA 40 downhole.Subtracting out this distance may then yield an estimated distancebetween the drill bit 42 and the bottom of the borehole 48.

FIG. 5 depicts a flow chart 60 outlining the process to calculate thedistance to the bottom of the borehole 48. The data processing system 28of the acoustic tool 12 may send a signal to the wave source 46 to emitone or more Stoneley waves (process block 62). The wave sensor 44 maythen detect the Stoneley wave(s) traveling down the borehole (processblock 64) and then again when the Stoneley wave(s) are traveling back upthe borehole (process block 66). The sensor signals may then be sent tothe data processing system 28 for calculating the distance to the bottomof the borehole 48 (process block 68).

As will be appreciated, the Stoneley waves may encounter multipleinterface changes along the wall of the borehole 16 before and after themultiple detections. These interface changes, such as fractures in theborehole 16, may cause other reflections in addition to the reflectionof the Stoneley wave pulse at the bottom of the borehole 48.Additionally, interface changes may also change the amplitude/intensityof the Stoneley waves, as part of the wave may be transmitted past theinterface, while the other part is reflected. Furthermore, compressionaland/or shear waves may also be produced as a result of the Stoneleywaves interacting with the borehole 16. As such, the wave sensor 44 maydetect multiple waves in addition to the expected Stoneley wave. Whendetermining the roundtrip time of a Stoneley wave, the data processingsystem 28 may take these and other factors into account, for example bydisregarding (e.g., filtering) the compressional/shear waves or othersignals corresponding to discontinuities in the borehole.

In one embodiment, the acoustic tool 12 may include two wave sensors,44A and 44B, as shown in FIG. 6. Using two or more wave sensors 44 mayassist accurate detection of the Stoneley waves and improve theidentification of which Stoneley wave corresponds to each emission. Forexample, using two or more sensors may allow the data processing system28 to ascertain a direction for a detected wave.

As shown in the time diagram 70 illustrated in FIG. 7, multipledetections of the same Stoneley wave may be detected on each leg of theroundtrip. As with a previous embodiment, time 72 shows a Stoneley wavepulse that may include two pulses, one traveling up the borehole 16towards the surface and one traveling down the borehole 16 towards thebottom of the borehole 48. As depicted at time 74, the downwardtraveling Stoneley wave pulse may be detected by the wave sensor 44A.After passing the first wave sensor 44A the Stoneley wave pulse thenpasses, and is detected by, the second wave sensor 44B, at time 76. TheStoneley wave then travels along the wall of the borehole 16 until it isreflected at the bottom of the borehole 48 as shown at time 78.Afterwards, at time 80, the Stoneley wave pulse travels back up theborehole 16 to be detected again by the wave sensor 44B, and then againby wave sensor 44A, at time 82.

The direction of travel for a detected wave may be inferred by firstdetecting the wave at one wave sensors (44A or 44B) and subsequentlysensing the wave at a different wave sensor (44B or 44A). For example,sensing at wave sensor 44A first and then at wave sensor 44B wouldindicate a downward traveling wave. As will be appreciated, the speed ofthe Stoneley wave may also be calculated by noting the time betweendetection at the multiple wave sensors 44.

As stated above, multiple waves may be present in the borehole 16. Thesewaves may be emitted as part of a wave train, as compressional or shearwaves, or as reflections and/or transmissions thereof. Additionally, thewaves may superimpose on each other. To assist in overcoming thedifficulties these multiple waves may present, in one embodiment, apulse sequence, such as a pseudo-random or pseudo-randomauto-correlating pulse sequence, may be employed to help distinguishbetween the multiple wave pulses passing the wave sensors 44 in theborehole 16.

A pseudo-random sequence may have a series of wave pulses that areoutput at seemingly random time intervals. However, at the end of aperiod, the entire sequence may be repeated. These sequences may bebased, at least in part, on a primitive polynomial or other function.Because of the pseudo-randomness of the output Stoneley wave pulses, theinterference of the multiple waves may be filtered and/or reduced.Additionally, the auto-correlation of the output Stoneley wave pulseswith their associated pseudo-random output times may assist in keepingtrack of which detection belongs to which Stoneley wave. Thiscorrelation may be done using an auto-correlation function, and, thus,may lead to a more accurate measurement of distance to the bottom of theborehole 48.

Flow chart 90, as shown in FIG. 8 describes another embodiment of aprocess to calculate the distance to the bottom of the borehole 48,beginning with emitting Stoneley waves in a pulse sequence (e.g., apseudo-random, auto-correlating pulse sequence) (process block 92). Insome embodiments, the data processing system 28 may control the emissionof the Stoneley waves in the pseudo-random, auto-correlating sequence.The Stoneley waves may then be detected by the multiple wave sensors 44(process block 94). Using the signals from at least one of the multiplewave sensors 44, the data processing system 28 may calculate anestimated Stoneley wave speed (process block 96). As the Stoneley wavesmake their way back up the borehole 16, they once again pass themultiple wave sensors 44 and are detected (process block 98). The dataprocessing system 28 may calculate the distance to the bottom of theborehole 48 (process block 100). For example, the data processing system28 may use an auto-correlation function, combined with the known emittedsequence, to distinguish the Stoneley wave detections. Additionally, thedata processing system 28 may take the calculated distance and use themeasurement to control or provide feedback for the descent of the BHA40, and thus reduce, minimize, and/or optimize its impact at the bottomof the borehole 48 (process block 102).

As will be appreciated, the measurements taken by the acoustic tool 12,as described above, may be taken while the BHA 40 is stationary or inmotion. However, using measurements taken at multiple depths within theborehole 16 may lead to a more accurate calculation of the speed of theStoneley waves, and thus the overall calculation of the distance to thebottom of the borehole 48. In the previous examples, the estimation ofthe Stoneley waves' speed gave the speed of the Stoneley waves in thesection of the borehole 16A that is adjacent to the BHA 40 and/or thetool string as depicted in FIG. 9. However, the Stoneley waves maypropagate at a different speed in a section of the borehole 16B that isvacant (i.e., is not adjacent to the BHA 40 or the tool string). Assuch, taking into account the speed in both sections of the borehole 16Aand 16B may further increase the accuracy of the distance measurement.

The calculation of the distance to the bottom of the borehole 48 and thespeeds of the Stoneley waves may be solved simultaneously in a system oflinear equations by defining certain variables/known quantities. Forexample, variables may include a starting distance 110 between the drillbit 42 and the bottom of the borehole 48, a distance traveled 112 whenmoving the acoustic tool 12 to a new location, and a tool distance 114between the drill bit 42 and the wave sensor 44B, the measured roundtriptime at the 1^(st) position, the measured roundtrip time at the 2^(nd)position, and the speeds in the two sections of the borehole 16A and16B. As will be appreciated, the wave sensor 44A may also be used withthe proper adjustments to the equations.Roundtrip time at 1^(st) position=2×(Distance 110)/(Speed in16B)+2×(Distance 114)/(Speed in 16A)  (EQ I)Roundtrip time at 2^(nd) position=2×((Distance 110)−(Distance112))/(Speed in 16B)+2×(Distance 114)/(Speed in 16A)  (EQ II)

When using the above equations, the distance traveled 112 and tooldistance 114 are known quantities and the speed of the Stoneley waves inthe section of the borehole 16A that is adjacent to the tool may becalculated as explained above, leaving two equations and two unknowns.As will be appreciated, multiple measurements at multiple depths in theborehole 16 may be used to increase the accuracy of the distancemeasurement, each new measurement adding another equation. Additionally,if three or more measurements are taken, the speed of the Stoneley wavein the section of the borehole 16A that is adjacent to the tool may alsobe calculated via the system of linear equations. Furthermore, becausewave sensor 44A may have a different depth in the borehole 16 than wavesensor 44B, a single location of the BHA 40, may yield an equation foreach of the multiple wave sensors 44 of the acoustic tool 12.

One embodiment is shown in the flow diagram 120 of FIG. 10, beginningwith a first Stoneley wave being emitted (process block 122). This maytake the form of a pulse, a wave train, a pseudo-random sequence ofpulses, a continuous waveform, or a combination thereof. As the firstStoneley wave passes the multiple wave sensors 44, the wave is detected(process block 124), and the speed of the Stoneley wave in the sectionof the borehole 16A that is adjacent to the BHA 40 may be calculated(process block 126). The Stoneley wave is then reflected off of thebottom of the borehole 48, and detected again by the multiple wavesensors 44 (process block 128). The data processing system 28 may thencoordinate the movement of the BHA 40 to a new depth in the borehole 16(process block 130). The emission and detection of a second Stoneleywave and its reflection is then repeated at this new depth (processblocks 132, 134, and 136). The speed of the Stoneley waves in thesection of the borehole 16B that is vacant may then be calculated(process block 138), along with the distance to the bottom of theborehole 48 (process block 140). Once the distance has been calculated,the data processing system 28 may then provide feedback for lowering theBHA 40 to the bottom of the borehole 48 (process block 142). Forexample, the data processing system 28 may control the lowering speed ofthe BHA. As will be appreciated, moving the BHA 40 may be operated by aseparate controller, and the data processing system 28 may simplyprovide measurement data instead of actively controlling the movement.Furthermore, although the steps of flow charts 60, 90, and 120 have beendepicted in an order, as will be appreciated, in certain embodiments,portions of the flow charts may be reordered, deleted, and/or occursimultaneously.

The specific embodiments described above have been shown by way ofexample, and it should be understood that these embodiments may besusceptible to various modifications and alternative forms. It should befurther understood that the claims are not intended to be limited to theparticular forms disclosed, but rather to cover modifications,equivalents, and alternatives falling within the spirit and scope ofthis disclosure.

The invention claimed is:
 1. A downhole tool system comprising: aStoneley wave emitter in a downhole tool, wherein the Stoneley waveemitter is configured to emit Stoneley waves into a borehole; one ormore Stoneley wave sensors in the downhole tool; and a processorconfigured to receive signals from the one or more Stoneley wave sensorsbased on a detection of the Stoneley waves, use the signals to obtain atemporal measurement of the Stoneley waves, and calculate a distancefrom the downhole tool or a bottom-hole assembly to a bottom of theborehole based at least in part on the temporal measurement, and whereinthe processor is configured to disregard signals corresponding todiscontinuities in a wall of the borehole.
 2. The downhole tool systemof claim 1, wherein the temporal measurement comprises a time from afirst sensing of a Stoneley wave to a second sensing of the Stoneleywave after the Stoneley wave has reflected off the bottom of theborehole.
 3. The downhole tool system of claim 1, wherein the processoris configured to use the signals to calculate a speed of the Stoneleywaves in the borehole.
 4. The downhole tool system of claim 1, whereinthe one or more Stoneley wave sensors comprises at least two Stoneleywave sensors.
 5. The downhole tool system of claim 4, wherein theprocessor is configured to calculate a direction of travel of theStoneley waves based at least in part on the signals from the at leasttwo Stoneley wave sensors.
 6. The downhole tool system of claim 1,wherein the processor is configured to control the Stoneley wave emitterto emit a pseudo-random, auto-correlating pulse sequence and calculatethe distance from the downhole tool or the bottom-hole assembly to thebottom of the borehole based at least in part on a correlation of theemitted pseudo-random, auto-correlating pulse sequence and the signalsfrom the one or more Stoneley wave sensors.
 7. The downhole tool systemof claim 1, wherein the processor is configured to obtain a plurality oftemporal measurements corresponding to a plurality of distances awayfrom the bottom of the borehole.
 8. The downhole tool system of claim 7,wherein the processor is configured to use the plurality of temporalmeasurements to calculate a speed of Stoneley waves in the borehole nextto the downhole tool and a speed of Stoneley waves in the boreholebeyond the downhole tool.
 9. The downhole tool system of claim 8,wherein the processor is configured to use the plurality of temporalmeasurements, the speed of Stoneley waves next to the downhole tool, andthe speed of Stoneley waves in the borehole beyond the downhole tool tocalculate the distance from the downhole tool or a bottom-hole assemblyto a bottom of the borehole.
 10. The downhole tool system of claim 1,wherein the Stoneley waves comprise wave pulses.
 11. The downhole toolsystem of claim 1, wherein the processor is configured to disregardsignals corresponding to compressional or shear waves.
 12. The downholetool system of claim 1, wherein the Stoneley wave emitter is configuredto emit Stoneley waves substantially continuously, and the processor isconfigured to substantially continuously calculate the distance from thedownhole tool or a bottom-hole assembly to the bottom of the boreholewhile the downhole tool approaches the bottom of the borehole.
 13. Amethod comprising: emitting, from a downhole tool in a bottom-holeassembly comprising a drill bit, a plurality of Stoneley waves into aborehole in a pulse sequence, while the drill bit is off a bottom of theborehole and is being lowered to the bottom of the borehole; detecting,via a plurality of sensors, the plurality of Stoneley waves traveling inthe borehole; and calculating, via a processor, a distance from thedownhole tool to the bottom of the borehole based at least in part onthe speed of the plurality of Stoneley waves in the borehole and a timebetween detections of the plurality of Stoneley waves.
 14. The method ofclaim 13 comprising reducing or minimizing an impact of the downholetool with the bottom of the borehole based at least in part on thecalculated distance from the downhole tool to the bottom of theborehole.
 15. The method of claim 13, wherein the pulse sequence is apseudo-random, auto-correlating pulse sequence, and wherein calculatingthe distance from the downhole tool to the bottom of the boreholecomprises correlating the pseudo-random, auto-correlating pulse sequencewith a detected plurality of Stoneley waves traveling up a wall of theborehole after reflecting off the bottom of the borehole.
 16. The methodof claim 13, wherein the plurality of Stoneley waves comprises a firstplurality of Stoneley waves, and wherein the method comprises: movingthe downhole tool a known distance to a new position in the borehole;and emitting and sensing a second plurality of Stoneley waves into theborehole at the new position.
 17. The method of claim 16, comprisingcalculating, via the processor, a speed of the first plurality ofStoneley waves while propagating along a wall of the borehole whileadjacent to the downhole tool and a speed of the first plurality ofStoneley waves while propagating along the wall of the borehole whilenot adjacent to the downhole tool.
 18. Machine-executable instructionsstored on a tangible, non-transitory machine readable storage medium,which, when executed by a machine, causes the machine to perform amethod, the method comprising: controlling an emission of one or moreStoneley waves at a plurality of locations in a borehole; processingsignals received from one or more Stoneley wave sensors, wherein thesignals are representative of the one or more Stoneley waves sensedwithin the borehole; calculating a direction of travel of the one ormore Stoneley waves; calculating a roundtrip time of the one or moreStoneley waves propagating down the borehole and reflecting back to theone or more Stoneley wave sensors based at least in part on thedirection of travel of the one or more Stoneley waves; calculating adistance from a bottom-hole assembly to the bottom of the borehole basedat least in part on the roundtrip time of the one or more Stoneleywaves; calculating a first speed of Stoneley waves corresponding to theone or more Stoneley waves propagating adjacent to the bottom-holeassembly; and calculating a second speed of Stoneley waves correspondingto the one or more Stoneley waves propagating in a section of theborehole not adjacent to the bottom-hole assembly.